That color in the Pro-100 print is typical of either a sepia-toned print or fading of dye black. Neither would surprise me. Mark McCormick thinks it could be "ink migration". I usually trust his opinion on matters of fading, but in this case I think he made a mistake.

Most black dye inks I have tested in the recent past fade to brown and finally to a dirty yellow and a few inks fade to a blueish gray which does not shift the overall color balance of an image that much as adding a suntan brown into it. I don't know whether it could be as well ink migration, that should be an effect very much depending on the type of coating - on some you almost can wash the ink away and other papers appear to be pretty waterproof, and here I would expect the dye molecules pretty much fixed in the resin (my assumption). I have not seen any specific tests - microscopic whatever - evaluating the possible effects of this migration within the coating layer, humidity is accelerating the fading process which may trigger this migration effect - I'm just speculating, fading is probably both - a physical effect like ink migration and a chemical reaction destroying/changing the dye molecules into some other with a different color triggered by UV, ozone or some other agent.
Both the Aardenburg lab and the Wilhelm institute have tested a wide range of papers with Canon, HP and Epson pigment inks, and the results vary widely for the different papers and inks.I would assume that pigment inks would not show the effect of ink migration due to the bigger pigment particle size so there must be some other effect related to the papers and its coatings causing the widely different results for the fading stability .

Hard to tell what is the culprit(s) with the black in fade based on the information given so far. It could be the ink, the paper, and/or the environment. We don't know the situation with his inkset (age, properly filled cartridges, contaminants, etc?). We do not know if this particular batch of paper was stored or if had a problem like this before. We do not know if any other prints in farmersteve's home are fading.

McCormick's suggestion to reprint and store hermetically sealed in the dark may exclude environment. Using other papers may exclude paper, Rechecking printer settings and making sure the proper inks are in their proper cartridges, and, getting a fresh batch of inks or an entirely different inkset may rule out the inks. A lot of variables to consider.

The biggest problem with using 3rd party inks is no two guys use it the same way or with the same paper so there can never be a credible test done on any of these inks, unlike OEM.

OEM ink works beautiful and best on its own branded papers and in an ideal world that’s what all of us would love to be using, but for the outrageous costs.

With 3rd party inks it is subject to all sort of contaminations from the home user, for a start its can be mixed with OEM ink, Water, Windex Detergent Ammonia, Acetone and a pleather of other solutions that we probably never even heard of, not to mention hygiene, any one of these would effect longevity.

Then we move onto paper again there are so many papers available to print on that are not made or suitable for 3rd party inks, so to compare the longevity of all the 3rd party inks with OEM is grossly unfair.

There are so many variables working again 3rd party inks, including very poorly made formulas and none of the OEM ink makers actually test their inks on any of the cheaper alternative papers available that we use.

Here is an example of Papilio vinyl paper in action.

I printed this four years ago using I.S. pigment inks but a couple of months ago I touched up the eyes with some enamel paint because it began to fade, but not the mouth.
So four years on and even using pigment inks, fading is evident on this type of paper, Now can I ask:- would OEM ink fair out any better in this situation, I won’t give any comment..

After reviewing my tests, I think his result is real, although I am surprised the light was that bright in his kitchen. On printer paper, a few days in the sun will do that to all the third-party dye blacks I have tested. Photo paper is somewhat better, but not immune to fading.

Oh well, we have evaluated here in very much detail the fading of inks for Epson printers, it appears that Canon users are left out from from decent inks in regards to fading. You can use original Canon inks, they are far superior to any 3rd party inks, but that's probably not really what you would like to do...

In my pixma I had some "standard" dye inks like sudhaus, tintenalarm, arici (german suppliers) - all fading very soon. The best of the bad came from MIS, which are possibly similar to PC, supposed both are made by or coming from IS.
Except these I tried also two different sorts of "OEM" inks: canon BCI-1411 (bigger carts for some canon dye LFP) and "GI-490", canon's bottled ink for the new home printers with integrated ciss.
While the BCI is fading at least like the "best of the bad" from above, the GI-490 is a disappointment to me: fading as fast as the "standard" dyes.
So there seems to be no such replacement for canon OEM dye ink like we have for epson OEM dye: fuji DL, epson D6, also L800 bottled ink shows decent UV resistance.

Do you have any further information, did I miss a test? As far as I know you prefer epson piezo over canon bubble jet...
A fading test of lyson fotonic XG by aardenburg, but for epson (!), shows one of the worst results you can find at aardenburg...